Policyholder Account Balance (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025		Jun. 30, 2024		Dec. 31, 2024
Policyholder Account Balance [Roll Forward]
Policyholders' Account Balance	$ 5,633.3				$ 5,633.7
Operating Segments
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	5,470.0		$ 5,514.2
Policyholder Account Balance, Premium Received	75.8		76.7
Policyholder Account Balance, Policy Charge	(118.2)	[1]	(112.8)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(44.9)		(43.7)
Policyholder Account Balance, Benefit Payment	(109.1)		(103.2)
Policyholder Account Balance, Interest Expense	191.5		165.9
Policyholder Account Balance, Increase (Decrease) from Other Change	4.4		5.3
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	5,469.5		5,502.4
Policyholder Account Balance, Reserves in excess of Account Balance	163.8		157.9
Policyholders' Account Balance	5,633.3		5,660.3
Policyholder Account Balance, Reinsurance recoverable	4,104.6		4,122.3
Policyholder Account Balance, After reinsurance recoverable	$ 1,528.7		$ 1,538.0
Policyholder Account Balance, Weighted Average Crediting Rate	7.30%		6.20%
Policyholder Account Balance, Net Amount at Risk	$ 13,527.8	[3]	$ 14,512.4	[4]
Policyholder Account Balance, Cash Surrender Value	5,406.8		5,437.8
Unum US | Voluntary Benefits
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	568.8		578.6
Policyholder Account Balance, Premium Received	25.2		27.3
Policyholder Account Balance, Policy Charge	(27.5)	[1]	(28.8)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(16.5)		(16.8)
Policyholder Account Balance, Benefit Payment	(3.1)		(3.3)
Policyholder Account Balance, Interest Expense	10.2		10.6
Policyholder Account Balance, Increase (Decrease) from Other Change	4.1		5.1
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	561.2		572.7
Policyholder Account Balance, Reserves in excess of Account Balance	109.9		101.8
Policyholders' Account Balance	671.1		674.5
Policyholder Account Balance, Reinsurance recoverable	0.8		0.9
Policyholder Account Balance, After reinsurance recoverable	$ 670.3		$ 673.6
Policyholder Account Balance, Weighted Average Crediting Rate	3.70%		3.70%
Policyholder Account Balance, Net Amount at Risk	$ 3,953.7	[3]	$ 4,300.4	[4]
Policyholder Account Balance, Cash Surrender Value	550.7		562.0
Colonial Life
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	849.0		852.9
Policyholder Account Balance, Premium Received	38.1		40.5
Policyholder Account Balance, Policy Charge	(35.0)	[1]	(36.4)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(19.4)		(20.4)
Policyholder Account Balance, Benefit Payment	(4.2)		(4.0)
Policyholder Account Balance, Interest Expense	17.0		17.1
Policyholder Account Balance, Increase (Decrease) from Other Change	0.0		0.0
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	845.5		849.7
Policyholder Account Balance, Reserves in excess of Account Balance	13.0		14.7
Policyholders' Account Balance	858.5		864.4
Policyholder Account Balance, Reinsurance recoverable	0.1		0.0
Policyholder Account Balance, After reinsurance recoverable	$ 858.4		$ 864.4
Policyholder Account Balance, Weighted Average Crediting Rate	4.10%		4.10%
Policyholder Account Balance, Net Amount at Risk	$ 7,924.6	[3]	$ 8,461.9	[4]
Policyholder Account Balance, Cash Surrender Value	821.2		816.4
Closed Block | Other Insurance Product Line
Policyholder Account Balance [Roll Forward]
Policyholder Account Balance, before Reserves in excess of Account Balance, Beginning of Year	4,052.2		4,082.7
Policyholder Account Balance, Premium Received	12.5		8.9
Policyholder Account Balance, Policy Charge	(55.7)	[1]	(47.6)	[2]
Policyholder Account Balance, Surrender and Withdrawal	(9.0)		(6.5)
Policyholder Account Balance, Benefit Payment	(101.8)		(95.9)
Policyholder Account Balance, Interest Expense	164.3		138.2
Policyholder Account Balance, Increase (Decrease) from Other Change	0.3		0.2
Policyholder Account Balance, before Reserves in excess of Account Balance, End of Period	4,062.8		4,080.0
Policyholder Account Balance, Reserves in excess of Account Balance	40.9		41.4
Policyholders' Account Balance	4,103.7		4,121.4
Policyholder Account Balance, Reinsurance recoverable	4,103.7		4,121.4
Policyholder Account Balance, After reinsurance recoverable	$ 0.0		$ 0.0
Policyholder Account Balance, Weighted Average Crediting Rate	8.40%		7.00%
Policyholder Account Balance, Net Amount at Risk	$ 1,649.5	[3]	$ 1,750.1	[4]
Policyholder Account Balance, Cash Surrender Value	$ 4,034.9		$ 4,059.4

[1] 1Contracts included in the policyholders' account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[2] 1Contracts included in the policyholders' account balances are generally charged a premium and/or monthly assessments on the basis of the account balance.

[3] 2For those guarantees of benefits that are payable in the event of death, the net amount at risk is generally defined as the current guaranteed minimum death benefit in excess of the current account balance at the balance sheet date.

[4] 2For those guarantees of benefits that are payable in the event of death, the net amount at risk is generally defined as the current guaranteed minimum death benefit in excess of the current account balance at the balance sheet date.